Preparation And Properties Of CaBi₂Nb₂O₉-Based Bismuth-Layered High-temperature Piezoelectric Ceramics

Piezoelectric ceramics can be exchanged between mechanical energy and electrical energy as the functional materials.They have wide application in aerospace,nuclear power,rail transportation,information communication and other fields.With the rapid development of new technologies,functional materials face more complex and extreme working environment,such as high-temperature environment application,radiation environment application,etc.The high-temperature piezoelectric ceramic is a material with high Curie temperature,which is expected to realize high temperature application.Ca Bi₂Nb₂O₉ ceramics possesses the highest Curie temperature（T_C～943°C）among the bismuth-layer structured piezoelectrics materials reported currently,and is an important candidate material for high-temperature piezoelectric applications.However,the spontaneous polarization along the a-b plane of Ca Bi₂Nb₂O₉ ceramics is small and the coercive electric field（E_c）is high,resulting in its piezoelectric coefficient(d₃₃)of only 6 p C/N.In addition,the volatilization of Bi³⁺in Ca Bi₂Nb₂O₉ceramics at high temperature will form a large number of oxygen vacancies(^··)and hole（?^·）carriers,resulting in a rapid decrease in resistivity（ρ）.At the same time,the sample will face greater breakdown risk in polarization.These factors severely limit the application of ceramics in high-temperature piezoelectric vibration sensors.Aiming at the problems of poor piezoelectric performance and low high temperature resistivity of Ca Bi₂Nb₂O₉ ceramic used in high temperature piezoelectric vibration sensor.Defect control and process adjustment are used as means to synergistically improve the piezoelectric properties and high temperature resistivity of Ca Bi₂Nb₂O₉ ceramics.The effects of ion doping and atmosphere sintering on the structure and properties of Ca Bi₂Nb₂O₉ ceramics were investigated.Its main research progress are as follows:（1）Ca Bi₂Nb_2-xW_xO₉ series ceramics were prepared by solid-phase method,and the W⁶⁺ion-doped high-temperature electrical conduction mechanism in Ca Bi₂Nb₂O₉ceramics were studied.It can neutralize hole carriers and effectively reduce oxygen vacancies by W⁶⁺donor substitution of Nb⁵⁺.At 600°C,the resistivityρis 2 orders of magnitude higher than that of pure Ca Bi₂Nb₂O₉ ceramics.The doping of W⁶⁺delays the diffusion of grains into the Ca Bi₂Nb₂O₉ lattice,and the grain size is reduced from7μm to 3μm,making it more uniform and dense.And its piezoelectric properties are significantly improved.After annealing at 900°C for 4 h,The piezoelectric coefficient can maintain 89.6%of the room temperature state.（2）Based on the composition design of Ca Bi₂Nb_1.975W_0.025O₉,a series of ceramics were prepared by doping Ce Ce⁴⁺.The effect of Ce⁴⁺and W⁶⁺composite co-doping on grain isotropy was elucidated.It is found that the degree of ceramic anisotropy is gradually reduced,inhibiting the growth of flake grains,and the ceramic grains are gradually transformed from flakes to granular,more refined and uniform.And the piezoelectric performance was further improved,reached 16.8 p C/N.（3）Using the Ddifferent atmosphere sintering process to prepare high-density Ca Bi₂Nb_1.975W_0.025O₉ piezoelectric ceramics by sintering in argon,air and oxygen.The effect of oxygen partial pressure on defect carriers of ceramics was studied,inhibited the formation of oxygen vacancy by oxygen sintering.The densification process of ceramic in oxygen sintering process was analyzed,the density of ceramic was increased to 99%,and the polarization breakdown electric field reached 240k V/cm.The signal verification of sensor device was carried out,which laid an important foundation for the practical application of the piezoelectric ceramics in the field of batch polarization and high temperature.